Show grabby hand cursors when hovering and dragging draggable widgets

Summary:
Same idea as D25998, but for desktop widgets and done on the C++ side to avoid weird
interactions between a MouseArea and a MultiPointTouchArea on the QML side.

Test Plan: {F7819381}

Reviewers: #vdg, #plasma, mart, apol

Reviewed By: #vdg, #plasma, mart, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26015